In winters 1990-1991 and 1991-1992 women with and without seasonal affective disorder, winter type, were treated by light at 2500 lux either in the morning (0800h-1000h) or afternoon (1600h-1800h). In winter before light treatment, melatonin levels in serum in daytime (1200h and 1600h) were higher in patients compared to controls (p < 0.05). This difference disappeared in the summer or after light treatment in the winter. Also, light treatment and change in season resulted in a phase advance shift of melatonin rhythm in patients. The decline in melatonin levels correlated with the decline in specific SAD symptoms of hyperphagia and carbohydrate craving. In winter, neither patients nor controls showed significant diurnal variations in levels of whole blood serotonin. In both patients and controls, levels of serotonin were higher in summer as compared with winter, especially at 2000h. Our data suggest that elevated daytime melatonin can be a state marker of winter depression, and that seasonal change of photoperiod may also affect the circadian amplitude and daytime levels of blood serotonin.